Abstract
A statistical combination of several searches for the electroweak production of charginos and neutralinos is presented. All searches use proton-proton collision data at sqrt{s}=13 TeV, recorded with the CMS detector at the LHC in 2016 and corresponding to an integrated luminosity of 35.9 fb−1. In addition to the combination of previous searches, a targeted analysis requiring three or more charged leptons (electrons or muons) is presented, focusing on the challenging scenario in which the difference in mass between the two least massive neutralinos is approximately equal to the mass of the Z boson. The results are interpreted in simplified models of chargino-neutralino or neutralino pair production. For chargino-neutralino production, in the case when the lightest neutralino is massless, the combination yields an observed (expected) limit at the 95% confidence level on the chargino mass of up to 650 (570) GeV, improving upon the individual analysis limits by up to 40 GeV. If the mass difference between the two least massive neutralinos is approximately equal to the mass of the Z boson in the chargino-neutralino model, the targeted search requiring three or more leptons obtains observed and expected exclusion limits of around 225 GeV on the second neutralino mass and 125 GeV on the lightest neutralino mass, improving the observed limit by about 60 GeV in both masses compared to the previous CMS result. In the neutralino pair production model, the combined observed (expected) exclusion limit on the neutralino mass extends up to 650–750 (550–750) GeV, depending on the branching fraction assumed. This extends the observed exclusion achieved in the individual analyses by up to 200 GeV. The combined result additionally excludes some intermediate gaps in the mass coverage of the individual analyses.
Highlights
If the mass difference between the two least massive neutralinos is approximately equal to the mass of the Z boson in the chargino-neutralino model, the targeted search requiring three or more leptons obtains observed and expected exclusion limits of around 225 GeV on the second neutralino mass and 125 GeV on the lightest neutralino mass, improving the observed limit by about 60 GeV in both masses compared to the previous CMS result
This paper implements a statistical combination of the searches performed by CMS in refs. [38,39,40,41,42,43] covering several final states to improve upon the sensitivity of the individual analyses, in models where the neutralino has a nonzero branching fraction to both Z and Higgs bosons
Cross section calculations to next-to-leading order (NLO) plus next-to-leadinglogarithmic (NLL) accuracy [54,55,56,57,58,59] in perturbative quantum chromodynamics (QCD) are used to normalize the signal samples for the results presented in sections 6 and 7
Summary
Simplified models of SUSY [44,45,46,47] are used to interpret the combined search results presented below. The production cross sections are computed in the limit of mass-degenerate winos χ±1 and χ02, and light bino χ01, with all other sparticles assumed to be heavy and decoupled. For bino- or wino-like neutralinos, the neutralino pair production cross section is very small, and we consider a specific gauge-mediated SUSY breaking (GMSB) model with quasidegenerate higgsi-. We present cross sections calculated to NLO accuracy [56] to demonstrate the dependence of the cross section values on assumptions made in decoupling other SUSY particles. The cross section calculation used in the interpretations of the analysis results assumes a mass of 100 TeV for the squarks and gluinos to have them fully decoupled. The dependence of the production cross section on the decoupling mass assumption was found to be much smaller in the higgsino case, at most a few percent, and it is small compared to the uncertainty in the cross section calculation
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